1. Field of the Invention
The present invention relates to an arrayed waveguide grating or a waveguide device having a slab waveguide, a demultiplexer or a multiplexer which employs an arrayed waveguide grating or a waveguide device, and an optical communication system which employs an arrayed waveguide grating or a waveguide device or a demultiplexer or a multiplexer.
2. Description of the Related Art
As the volume of data to be transmitted over an optical fiber communication system increases, it is desired that the optical fiber communication system have an increased capacity for data transmission. In view of such a demand, growing importance is attached to optical wavelength filters for use as multiplexing and demultiplexing devices for multiplexing and demultiplexing wavelengths in DWDM (Dense Wavelength Division Multiplexing) communication systems.
Optical wavelength filters are available in various types. Of the various wavelength filters, an arrayed waveguide grating (AWG) has a narrow wavelength band and a high extinction ratio, and has features as a multi-input, multi-output filter device. The arrayed waveguide grating is capable of demultiplexing multiplexed signals and multiplexing signals, and can easily be used to make up wavelength multiplexing and demultiplexing devices.
FIG. 1 of the accompanying drawings shows an overall arrangement of a conventional arrayed waveguide grating. As shown in FIG. 1, the conventional arrayed waveguide grating, generally denoted by 11, comprises substrate 12, one or plural input waveguides 13 disposed on substrate 12, a plurality of output waveguides 14 disposed on substrate 12, channel waveguide array 15 disposed on substrate 12, and curved in a certain direction with respective curvatures, inlet slab waveguide 16 disposed on substrate 12, and connecting input waveguides 13 to channel waveguide array 15, and outlet slab waveguide 17 disposed on substrate 12, and connecting channel waveguide array 15 to output waveguides 14. Multiplexed signal light entered from input waveguides 13 is spread by inlet slab waveguide 16, and enters into channel waveguide array 15.
Channel waveguide array 15 comprises a plurality of arrayed waveguides having respective different optical path lengths which are successively longer or shorter. Therefore, signal light beams guided through the respective arrayed waveguides couple in respective different phases spaced at certain intervals to the outlet slab waveguide 17. Since the signal light beams actually suffer chromatic dispersion, the cophasal surfaces of the signal light beams are inclined depending on the wavelength. As a result, the signal light beams are focused (converged) at different positions corresponding to the difference wavelengths on the interface between the outlet slab waveguide 17 and the output waveguides 14. Since the output waveguides 14 are disposed in the respective positions corresponding to the difference wavelengths, desired wavelengths can be extracted from the respective output waveguides 14. The slab waveguides are disclosed in Japanese laid-open patent publication No. 7-63934, for example. The general technique of multiplexing and demultiplexing optical signals is disclosed in Japanese laid-open patent publication No. 7-49430, for example.
With the conventional arrayed waveguide grating 11 shown in FIG. 1, the light emitted from channel waveguide array 15 into outlet slab waveguide 17 reaches output waveguides 14 that is connected to the output side of outlet slab waveguide 17. In output waveguides 14, the intensity of the light is greater progressively toward the central ones of output waveguides 14, and smaller progressively toward the peripheral ones of output waveguides 14.
Heretofore, it has been proposed to uniformize the levels of the optical signals thereby to uniform the levels of the optical signals that are detected from the output waveguides. According to one proposal, in order to adjust the levels of the optical signals that are detected from the output waveguides, attenuators for compensating loss differences are individually connected to the respective output waveguides, thus making up an attenuator. However, it is necessary to prepare as many resistors having different resistances as the number of the different levels of the optical signals that are detected from the output waveguides. Furthermore, since the attenuation levels of the attenuators vary depending on the temperature, it is necessary to use a temperature compensation circuit in combination with the attenuators. The arrayed waveguide grating with such attenuators for compensating loss differences is not practical as to cost and space.
An arrayed waveguide grating which is designed to extract a monitor signal using higher-order diffracted light tends to cause a large detected light level difference because a waveguide for guiding the monitor signal is positioned away from the optical axis of light emitted from a channel waveguide array. Consequently, the arrayed waveguide grating needs a structure for compensating for a signal level loss before or after the signal light is detected.
Japanese laid-open patent publication No. 2000-98177 discloses a device having an optical waveguide with a plurality of ports and an optical fiber array. Deviations between the ports and the propagation axes of the optical fibers of the optical fiber array are managed to set transmission losses between the ports of the optical waveguide to desired values. Though the disclosed arrangement does not need an external attenuator, it poses yield and cost problems because of the need for fine adjustment of the propagation axes.
While the drawbacks of the conventional arrayed waveguide gratings have been described above, multiplexers for multiplexing optical signals and demultiplexers for demultiplexing optical signals which employ the conventional arrayed waveguide gratings, and optical communication systems which employ the conventional arrayed waveguide gratings and the multiplexers and demultiplexers are also problematic in that they are complex in structure and large in size, and cannot be reduced in cost.